Recently, electronic technology has been remarkably developed, and various appliances have been made smaller and lighter. Along with the miniaturization and reduction in weight of electronic appliances, miniaturization and reduction in weight of batteries, serving as power sources of these electronic appliances, have been demanded. As batteries that have small volume and mass but are capable of providing large amounts of energy, non-aqueous electrolyte secondary batteries using lithium have been used. In addition, it has been proposed to use non-aqueous electrolyte secondary batteries as power sources for hybrid cars, electric cars, and the like, and the non-aqueous electrolyte secondary batteries have been put into practical use.
Generally, a non-aqueous electrolyte secondary battery has a positive electrode, a negative electrode, and a separator provided therebetween for insulating the positive electrode and the negative electrode. Conventionally, a porous film of a polyolefin-based polymer has been used as a separator used in the non-aqueous electrolyte secondary battery.
In the non-aqueous electrolyte secondary batteries, due to ions (in the case of a lithium-ion secondary battery, lithium ion (Li+)) moving between a positive electrode and a negative electrode through a separator, charging and discharging are possible. Therefore, the separator is required to not inhibit ions from moving freely, and a porous film having a plurality of microscopic holes has been used as the separator.
In addition, the separator is required to have a so-called shutdown function. The shutdown function is a function that improves safety of the non-aqueous electrolyte secondary battery by, for cases where a fine short circuit has occurred in a battery, inhibiting the movement of ions by blocking the holes in the part where the short circuit occurred in order to make the battery lose the function at the part. In the porous film of a polyolefin-based polymer, the shutdown function is achieved by, for cases where a fine short circuit occurred in the battery, melting the part where the short circuit occurred by increasing the temperature and thereby blocking the holes.
However, since the separator formed from polyolefin-based polymer had low affinity toward electrolyte solutions, for cases where an electrolyte solution was retained in the separator, the electrolyte solution merely filled within the holes thereof, and therefore, the separator had a problem that the retention of electrolyte solutions was low. If the retention of electrolyte solutions was low, problems such as reduction in battery capacity, deterioration of cycle characteristics, and limitation on working temperature might arise. Furthermore, the adhesion of the polyolefin-based polymer toward other resins or other materials was poor, a gap was readily formed at an interface between the polyolefin-based polymer and an electrode. As a result, there were cases where reduction in battery capacity and deterioration of charge and discharge characteristics were caused.
Furthermore, a method for producing a lithium-ion secondary battery for the purpose of achieving both electrical connection and adhesion between positive and negative electrodes and a separator has been proposed (e.g. see Patent Document 1). In Patent Document 1, the object described above was achieved by providing an adhesive resin layer between the separator and the electrodes. Furthermore, it was proposed to use polyvinylidene fluoride as the adhesive resin. However, polyvinylidene fluoride was far from being a resin that exhibits excellent adhesion toward polyolefin constituting a separator.